Conventional equipment for the fermentation and propagation of micro-organisms generally consists of a vessel equipped with means for introducing gases such as oxygen, air, or carbon dioxide into the fermentation liquor, and a motor-driven mechanical mixing means to provide intense agitation. This agitation creates a large quantity of bubbles of the gaseous phase within the fermentation liquor, thus facilitating the molecular transport between the gaseous phase and the fermentation liquor. Typical mechanical mixing means consist of various designs of impellers to achieve efficient distribution of the gaseous phase into the fermentation liquor and to provide adequate flow characteristics of the liquid nutrient medium which is necessary for the physiology of the cultivated plant or animal cells. The mass transfer capabilities of such equipment can be improved by such modifications by the use of baffling systems, draft tubes, or air lift methods.
However, each of these conventional methods requires a great deal of mechanical energy to distribute the gaseous phase through the liquid phase and to provide the necessary mixing in large industrial scale equipment, resulting in both technical and economic problems. In addition to the high energy consumption of these systems, the mechanical agitation of the fermentation liquor in many fermenters creates high shear stresses on the cells which limit or inhibit cell growth.
Another problem with conventional fermenters concerns foaming. In prior art systems, the introduction of large quantities of gas into the vigorously agitated fermentation liquor often produces great quantities of foam in the reaction vessel. This foam severely limits the usable volume of the vessel and can render the fermentation process inoperable and microbially contaminated when the gas flow exit lines become filled with foam. All of these problems have a substantially adverse influence upon the yield an cost-effectiveness of conventional fermentation processes.
Numerous chemical and mechanical devices have been proposed to solve the foaming problem in industrial biosynthesis processes. Most of the existing methods are founded upon chemical or mechanical defoaming of a developed foam. Chemical treatment currently used for defoaming involves silicones and other water-immiscible additives which substantially decrease the rate of oxygen transfer, thus interfering with an effective process of aerobic biosynthesis. The mechanical defoamers which are sometimes used in fermentation processes typically require an additional power source and a particular fermenter design to accommodate the defoaming equipment. Mechanical defoamers are not uniformly reliable or feasible, especially in large fermentation vessels. In summary, the disadvantages of the prior art procedures for mass transfer in aerobic processes are as follows: high cost of mixing and aeration, heavy foaming, high shear stress on cells, and frequent incidents of contamination in aerated systems. These difficulties, all in their own way, interfere with the efficiency and economy of the fermentation processes.
Devices which eliminate or greatly diminish some of the above disadvantages of the prior art have been described in previous U.S. Pat. No. 4,339,398, to Feres, one of the inventors herein, and in the related U.S. Pat. No. 4,657,677, to the applicants herein. The equipment disclosed in the related application is for improved mass transfer achieved by generating a thin film of liquid which flows upward along a rotating truncated conical surface, exposing a large area of flowing liquid to a substantially static, gaseous phase. This equipment is useful in promoting efficient molecular transfer of gases with a low solubility in the liquid, for example the transport of oxygen into an aqueous phase, typical of conventional aerobic fermentation processes. The principle of this system can alternately be employed in the reverse direction of transfer, as is with gases leaving a liquid phase, such as occurs in stripping, defoaming, and deodorization. An important feature of this equipment, like the present invention, is the fact that the thin-film process prevents the formation of foam, a serious problem with prior art fermentation processes. Traditional prior art methods of foam control have been directed toward control of existing foam, not toward preventing the formation of foam.
It is desirable to produce equipment to efficiently and economically, from both an energy and process effectiveness standpoint, carry out fermentation processes. It is also desirable to produce a fermenter which is substantially free from microbial contamination and in which the process itself prevents the formation of foam. It is still further desirable to produce a fermenter which does not subject cells and organisms to high shear stress.
These and other undesirable problems of the prior art are overcome by the present invention and improved equipment for carrying out the fermentation process is provided.